Subsurface CO₂ storage in unconventional reservoirs: Insights into pore-scale characterization of geochemical interactions and particle migration

Unconventional reservoirs, such as the Bakken Formation, are one of the promising formations for enhanced oil recovery (EOR) and long-term subsurface Carbon Dioxide (CO₂) storage due to their wide availability and strong sealing capabilities. The physical properties of the rocks can undergo significant alteration during the CO₂ injection and soaking stage. While the interaction between brine, rock, and CO₂ has been widely studied, there is limited understanding of how the buffering capacity of carbonate-rich and silicate-rich rocks influences these interactions and alters pore-scale properties, fluid flow, brine chemistry, and mineralogical composition under the complex mineralogy of Bakken rocks. To address this problem, we present an experimental investigation on how supercritical CO₂ (Sc-CO₂) influences mineralogy, fluid flow, pore structure, and brine chemistry in carbonate-rich and silicate-rich samples from the Bakken Formations. The results show a dissolution behavior of carbonite minerals, specifically in the samples with a higher calcite content after the 38 days of CO₂ treatment. While clay minerals remained stable or even showed minimal precipitation. The buffered environment by dissolving calcite, consuming H^+, and releasing calcium ions (Ca²⁺) reduces the overall acidity of the system, effectively increasing the pH, and creating a less hostile setting for clay dissolution. Conversely, in the silicate-rich samples, clay minerals display dissolution behavior. The lower buffering capacity in these cores resulted in more aggressive acidic conditions, favoring clay dissolution and the release of silica and other cations that contributed to quartz precipitation. The T₂ relaxation time profiles indicate that macropores, medium and small pores are initially penetrated by CO₂ within 1, and 7 days due to their low capillary pressure, while micropores remained immobile or showed minimal reduction. However, extending the soaking time to 30 days was necessary for the soaking pressure to overcome the capillary pressure resistance within these pores. Moreover, a significant accumulation of brine in macropores is observed after 30 days of treatment due to the precipitation of salt within the pores, which significantly reduces fluid mobilization. Notably, this brine accumulation is more pronounced in samples with low buffering capacity. The brine chemistry analysis at the end of experiments (38 days) shows high concentrations of Ca⁺ (208 ppm) reflect the substantial dissolution of carbonate minerals, particularly calcite. In contrast, the low concentrations of Al³⁺ (7 ppm), indicate limited dissolution of aluminum silicate minerals. Although silicate breakdown is evidenced by an increase in Si⁴⁺ (80 ppm), the stability of aluminum-bearing silicate phases implies that these minerals are less reactive to acidic conditions. Electron micrographs display a significant increase of the surface roughness where the content of calcite was higher along with precipitation of salt crystals, suggesting that the pore spaces are filled with precipitated minerals, wich may reduce permeability and porosity.